Straightforward and reliable methods for simultaneously analyzing several constituents of a complex sample are extremely desirable. Polynucleotide arrays (such as DNA or RNA arrays) are known and are used, for example, as diagnostic or screening tools. Such arrays include regions of usually different sequence polynucleotides (“capture agents”) arranged in a predetermined configuration on a support. The arrays are “addressable” in that these regions (sometimes referenced as “array features”) have different predetermined locations (“addresses”) on the support of array. The polynucleotide arrays typically are fabricated on planar supports either by depositing previously obtained polynucleotides onto the support in a site specific fashion or by site specific in situ synthesis of the polynucleotides upon the support. After depositing the polynucleotide capture agents onto the support, the support is typically processed (e.g., washed and blocked for example) and stored prior to use.
In use, an array is contacted with a sample or labeled sample containing analytes (typically, but not necessarily, other polynucleotides) under conditions that promote specific binding of the analytes in the sample to one or more of the capture agents present on the array. Thus, the arrays, when exposed to a sample, will undergo a binding reaction with the sample and exhibit an observed binding pattern. This binding pattern can be detected upon interrogating the array. For example all target polynucleotides (for example, DNA) in the sample can be labeled with a suitable label (such as a fluorescent compound), and the label then can be accurately observed (such as by observing the fluorescence pattern) on the array after exposure of the array to the sample. Assuming that the different sequence polynucleotides were correctly deposited in accordance with the predetermined configuration, then the observed binding pattern will be indicative of the presence and/or concentration of one or more components of the sample. Techniques for scanning arrays are described, for example, in U.S. Pat. No. 5,763,870 and U.S. Pat. No. 5,945,679. Still other techniques useful for observing an array are described in U.S. Pat. No. 5,721,435.
Since the discovery of the biological activity of short interfering RNAs (siRNAs) over a decade ago, so called “small RNAs” (i.e., short non-coding regulatory RNAs that have a defined sequence) have become a subject of intense interest in the research community. See Novina et al., Nature 430: 161-164 (2004). Exemplary small RNAs include siRNAs, microRNAs (miRNAs), tiny non-coding RNAs (tncRNAs) and small modulatory RNAs (smRNAs), as well as many others.
Although the exact biological functions of most small RNAs remain a mystery, it is clear that they are abundant in plants and animals, with up to tens of thousands of copies per cell. For example, to date, over 78 Drosophila microRNA species and 300 human microRNA species have been identified. The levels of the individual species of small RNA, in particular microRNA species, appear to vary according to the developmental stage and type of tissue being examined. It is thought that the levels of particular small RNAs may be correlated with particular phenotypes, as well as with the levels of particular messenger RNAs and proteins. Further, viral microRNAs have been identified, and their presence has been linked to viral latency (see Pfeffer et al., Science, 304: 734-736 (2004)).
The sequences of several hundred miRNAs from a variety of different species, including humans, may be found at the microRNA registry (Griffiths-Jones, Nucl. Acids Res. 2004 32:D109-D111), as found at the world-wide website of the Sanger Institute (Cambridge, UK) (which may be accessed by typing “www” followed by “.sanger.ac.uk/cgi-bin/Rfam/mirna/browse.pl” into the address bar of a typical internet browser). The sequences of all of the microRNAs deposited at the microRNA registry, including more than 300 microRNA sequences from humans (see Lagos-Quintana et al, Science 294:853-858 (2001); Grad et al, Mol Cell 11: 1253-1263 (2003); Mourelatos et al, Genes Dev 16:720-728 (2002); Lagos-Quintana et al, Curr Biol 12:735-739 (2002); Lagos-Quintana et al, RNA 9:175-179 (2003); Dostie et al, RNA 9:180-186 (2003); Lim et al, Science 299:1540 (2003); Houbaviy et al, Dev Cell 5:351-358 (2003); Michael et al, Mol Cancer Res 1:882-891 (2003); Kim et al, Proc Natl Acad Sci USA 101:360-365 (2004); Suh et al, Dev Biol 270:488-498 (2004); Kasashima et al, Biochem Biophys Res Commun 322:403-410 (2004); and Xie et al, Nature 434:338-345 (2005)), are incorporated herein by reference. MicroRNAs (miRNAs) are a class of single stranded RNAs of approximately 19-25 nt (nucleotides) in length.
Analytic methods employing polynucleotide arrays have been used for investigating these small RNAs, e.g. miRNAs have become a subject of investigation with microarray analysis. See, e.g., Liu et al., Proc. Nat'l Acad. Sci. USA, 101: 9740-9744 (2004); Thomson et al., Nature Methods, 1:1-7 (2004); and Babak et al., RNA, 10: 1813-1819 (2004).
Polynucleotide arrays are also used in the characterization of cellular gene expression. Gene expression analysis involves detection of different mRNA species in a test population and the quantitative determination of different mRNA levels in that test population. Such analytic methods may be used to investigate differential expression between different tissue types, different stages of cellular growth or between normal and diseased states. In typical methods, an RNA sample from a test population and an RNA sample from a control population are distinguishably labeled. A polynucleotide array is then contacted with both test and control RNA samples. Ratios of the distinguishable labels detected on the polynucleotide array provide information on relative expression of different mRNA species in the test and control samples.
Methods of labeling RNAs are of interest for use in array analysis of RNA to provide an observable label used in interrogating the array. In the study of Liu et al., miRNA was transcribed into DNA with a biotin-labeled primer. This primer was subsequently labeled with streptavidin-linked Alexa dye prior to array hybridization. This method is susceptible to any reverse-transcriptase reaction bias. Further, the streptavidin-dye as well as streptavidin-biotin-RNA stochiometry may be difficult to quantify. In the study of Thomson et al., the miRNA was directly labeled with 5′-phosphate-cytidyl-uridyl-Cy3-3′ using T4 RNA ligase. This reaction is sensitive to the acceptor sequence. See England et al., Biochemistry, 17: 2069-2776 (1978). In the study of Babak et al (4), the miRNA was labeled with Ulysis Alexa Fluor system, which reacts with guanine residue (G) of RNA. Since different miRNAs do not have uniform G content, this method is not quantitative.
Thus, there is a continuing need for methods of labeling RNA with an observable label. Such methods may be used in conjunction with analytical methods based on observing the label, such as array-based analysis of polynucleotides.